Interconnection systems for a distributed computer network are well known in the field, one such interconnection system is described in the following patents: U.S. Pat. No. 4,450,572, titled INTERFACE FOR SERIAL DATA COMMUNICATIONS LINK, in the names of Robert E. Stewart, John E. Buzynski and Robert Giggi; U.S. Pat. No. 4,490,785, titled DUAL PATH BUS STRUCTURE FOR COMPUTER INTERCONNECTION, in the names of William D. Strecker, David Thompson and Richard Casabona; U.S. Pat. No. 4,560,985, titled DUAL-COUNT, ROUND-ROBIN DISTRIBUTED ARBITRATION TECHNIQUE FOR SERIAL BUSES, in the names of William D. Strecker, John E. Buzynski and David Thompson; U.S. Pat. No. 4,592,072, titled DECODER FOR SELFCLOCKING SERIAL DATA COMMUNICATIONS, in the name of Robert E. Stewart; and U.S. Pat. No. 4,777,595, titled APPARATUS FOR TRANSFERRING BLOCKS OF INFORMATION FROM ONE NODE TO A SECOND NODE IN A COMPUTER NETWORK, in the names of William D. Strecker, Robert E. Stewart and Samuel Fuller. The interconnection system described in the above referred to patents essentially comprises a bus having a limited number of nodes with virtual circuits established between the nodes which, at higher levels of protocol, appear to be bilateral connections between every pair of nodes in the network. Quantities of data commonly referred to as data packets are transmitted over the bus according to a time-multiplexing communication protocol.
Such interconnection systems provide a very effective means for networking all types and combinations of computer system elements, such as processors, disk drives and tape drives. Nevertheless, such interconnection systems also have a number of shortcomings or deficiencies. First, such interconnection systems have heretofore been implemented with a passive coupler that limits the number of nodes in the network and, thus, the number of computer system elements that are attachable to the network. The coupler commonly in use contains merely coupling transformers and requires no electrical power. Such an arrangement supports a high transmission speed in the range of about 70 million bits per second. However, signal attenuation becomes a limiting factor and accordingly restricts the number of nodes that can be connected to the network. An additional limitation on coupler size and the number of nodes arises due to the construction of a coupler that provides a communication path between a transmit path from each node and receive paths of all nodes. As the number of nodes is increased, the number of internal connections increases accordingly, and the practical size of a usable coupler is quickly limited. For at least the above reasons, the interconnection system size commonly in use today is effectively limited to about 32 nodes.
A second deficiency of the known interconnection systems lies in the manner in which devices in general and mass storage devices in particular are coupled to the network. Intermediate server devices have heretofore been used to interface with a node on the bus and provide access to the bus for a number of storage devices, such as disk or tape drives, connected to it. The server performs all bus transmit and receive functions on behalf of the storage devices connected to it. The server advantageously occupies only one node even though multiple devices can use it for access to the bus. An ancillary benefit of a server is that it reduces cabling since it can be located in or adjacent to a cabinet containing multiple storage devices so that only one set of bus cables, associated with the one node to which the server interfaces, must run to the area having the server and the storage devices. However, the server is expensive hardware, and it introduces a delay that impedes the performance of both the storage devices using it as well as the network as a whole. To reduce cost and improve speed by eliminating a server, it has heretofore been required to use one node for each device and accordingly run a set of cables to each individual device. The use of a server has thus been a suitable way to expand the number of devices effectively coupled to the node-limited network and minimize cabling, but it has drawbacks in terms of cost and diminished performance.